A photodetector is an apparatus comprising one or a plurality of photodiodes, an amplifier, and an integrating circuit including an integral capacitance part, and may further comprise signal processing circuits subsequent thereto (see, for example, International Publication No. 02/12845 pamphlet). In this photodetector, a switch is provided between each photodiode and an input terminal of the integrating circuit, such that electric charges generated in response to light incident on the photodiode are accumulated in a junction capacitance part of the photodiode during when the switch is open, whereas, when the switch is closed, the electric charges accumulated in the junction capacitance part so far are fed to the input terminal of the integrating circuit by way of the switch, so as to be accumulated in the integral capacitance part of the integrating circuit. Then, the voltage corresponding to the amount of electric charges accumulated in the integral capacitance part is outputted from the integrating circuit. According to the voltages outputted from the integrating circuit in response to the respective amounts of electric charges generated in a plurality of photodiodes, the light incident on a photosensitive surface in which the plurality of photodiodes are arranged is detected. There is a case where the photodetector further comprises an A/D converter circuit for converting voltages (analog values) outputted from the integrating circuit into digital values. In this case, the incident light intensity is obtained as a digital value, which can further be processed by a computer or the like.